[MUSIC] So dear students in today's lecture we are going to discuss the factors that affect diffusion part one. So, in our previous lecture, we have seen this general form of diffusion coefficient or diffusivity, where you have a temperature independent constant term Do times this exponential term with an activation energy of diffusion here. So you have different activation energy terms. For your interest digital diffusion, we can see diffusion mechanism, where in your weakness the diffusion mechanism, you have this additional vacancy formation, and so P term, in your activation energy term. So, it is an exponential term with a temperature dependence, so you can easily tell. That if you do a low end treatment on either side of the equation, you're lowing diffusivity will be having a linear relationship with your temperature or inverse temperature, right? So if you draw this reciprocal or inverse temperature versus your diffusion coefficient, you are going to have straight lines here, right? And the slope will be corresponding to the activation energy for diffusion. So, in addition to temperature, if you look at the diffusion data, okay, have a lot of diffusion problems, you may see that other factors will affect your default severity as well. For example, the structure of your material for example, if you consider here, your carbon interstitial diffusion in different structures of iron, you will see that the activation energy or activation energy barrier for your carbon diffusion in your alpha ion is much smaller. One half just one half of your activation energy in gamma, right? Because gamma you have FCC more closely packed structure. We will talk about this effect later. Another interesting observation is that if you have some crystalline defects such as grain boundaries, you're actually lowering down your activation energy for diffusion. We'll also talk about this later. So, the first factor that affect diffusion we will look into some detail is temperature, which is the primary one most important factor that affects diffusion coefficient, because you have this exponential dependence, right? So, an example is here, if you consider the carbon diffusion in your FCC gamma iron, if you just increase your temperature by 100 degrees celcius, your diffusivity will tripled. So three times diffusivity high temperature than the relatively low temperature diffusivity. So temperature has a huge. Effect on diffusivity. So, another factor is the different type of solid solution systems. And as we have discussed in interstitial alloys, interstitial diffusion is usually much more. Quick than the substitution diffusion and substitution of alloys, because in the vacancy diffusion mechanism in substitutional alloys, you have an additional term of vacancy formation enthalpy right? So, the different types of solid solution system will also. Work for the same type of material, where for example, if you consider carbon and nickel diffusion in both gamma, and so FCC iron, the interstitial diffusion of carbon items will corresponds to much higher orders of magnitude higher diffusivity than the diffusivity of nickel in iron, right, that makes sense, because interstitial diffusion is easier. The crystal structure will affect the self diffusion in polymorphic crystal structures as well. For example, if you have a gamma and have FCC structure It is more closely packed. So compared with the BCC, our fine structure is diffusion is more difficult. And this is the diffusion data or self diffusion data for BCC, RFI and FCC gamma iron, and you can see that the differ with different activation energies. Interesting observation is that, if you look at the diffusion data for different crystal structures, there may there are some interesting observations, particularly the so called Melting temperature. Normalized activation energy for diffusion and this melting temperature diffusivity which is the diffusivity and melting temperature for different types of crystal structure. In each of particular crystal structure. You usually have very similar normalized activation energy for diffusion, as well as fairly similar diffusivity at melting temperature, okay. Why is that? Why is that? So, recall that your diffusion depends on migrating of items from one site to another. So, it involves the breaking and formation of chemical bonds. And you know that your melting temperature is actually a measure of your chemical bonding strengths of your material. And for the same crystal structure, you have similar coordination number, right? You have similar bonding type. So, no wonder for the same crystal structure elements, you will have similar diffusivity at this melting temperature. And if you normalize your activation energy with the melting temperature times this ideal gas constant, or Boltzmann constant you will end up with a similar name number, because your activation energy as well as your melting temperature are both measures of your chemical bonding strengths. So, this plot makes this comparison more illustrative right. So for different types of crystal structures their range for their melting temperature diffusivity for their normalized activation energy for into a certain range. Okay? So there's another figure showing the normalized activation energy. Okay. So again, this is just a comment of what we just have discussed. In terms of the reduced or normalized temperature, each crystal structure that the crystals will have similar normalized activation energy for diffusion. And even for substitutional alloys, the structure or the crystal structure of the solvent material will also play a significant role. For example, if you consider Ni diffusion in gamma ion and in alpha Fe(bcc). Their diffusivity can be orders of magnitude difference, because you have a different packing density of the solvent items. So the anisotropy of crystal structure will also play a role in determining the diffusion. Okay. So high symmetry crystals for example, the cubic crystals, the properties, and structure is fairly isotropic, so, this effect is not that significant. However for non-cubic crystals, such an isotropic effect will be very significant, and this is illustrated here. So for HCP, and Roma hero, and diamond cubic crystals you have different diffusion coefficient in different decisions, and this is certainly clear evidence of anisotropy in diffusion in these crystals. So in today's lecture, we have talked about the factors that affect diffusion. Thank you very much, see you again. [MUSIC]
